In a radiometric particle sorter the ore particles are arranged in a plurality of spaced lines with the individual particles in each line being spaced from one another. Each line of particles is directed along a counting channel over a succession of detectors which are housed in a lead shield and the radioactive counts recorded by the detectors for each particle are accumulated to obtain a measure of the radioactive content of the particle.
Ideally the particles pass down the centre line of the channel and so are exposed under identical conditions to the detectors. In practice, though, due to the requirements of a high tonnage throughput and the limitations of the particle feeding system together with the fact that the sorter must be capable of handling particles having a size range of 2:1 or possibly 3:1 and the necessity of having a counting channel width within the lead shielding of about 2 to 3 times the nominal maximum particle size to avoid pile-up and jamming of particles within the counting channel, many particles, particularly smaller ones, are laterally displaced from the centre line of the counting channel and consequently from the centre line of the scintillation detectors.
The scintillation detectors normally have a scintillation crystal of 75 mm diameter as maximum cross sectional dimensions for several reasons, including minimising the effect of the following and preceding particles at acceptable particle separations and keeping background count low to maintain sensitivity and selectivity for small low grade particles. Many particles are therefore considerably displaced laterally from the centre line of the detectors, and give a considerably reduced count compared to the same particles travelling over the centre line of the detectors, due to the inverse square law attenuation of radiation and also due to the effect of particle-detector geometry.
Certain sorting machines previously described or built have compensated for this effect by using a single plane projected area volume measuring device to measure the lateral offset of the particle from the centre line, and adjust the reading of the projected area to compensate for the offset in a data processor to derive the particle's grade, i.e. the measured projected area or apparent volume, is reduced to compensate for the lower counts which the particle gives due to its offset. While this method gives acceptable results if only high grade particles giving counts considerably above background counts are being handled, it is not satisfactory when sorting low grade particles with counts only slightly above background counts, as this correction for lateral displacement by reduction of the apparent mass can result in a barren particle accumulating a background level count being processed to appear as an ore particle.